Unreported World (Channel 4) is now into its 18th series of hunting down and highlighting what you have to hope are some of the most extreme examples of human misery and despair in various war-torn and poverty-stricken countries round the world. They have covered everything from the rape and disappearance of hundreds of women from Ciudad Juárez (in 2003's Mexico: The City of Lost Girls), and the truly soul-destroying exploitation of child soldiers in a number of desperate countries, to last year's investigation into the trade of body parts for use in South African traditional medicine.

Every half-hour episode is a miniature masterpiece, outlining a desperate situation in swift, sure strokes, and last night's Refugees for Sale was no exception.

Reporter Aidan Hartley followed members of the Malaysian government's militia, an army of volunteers known as RELA, who take to the streets every night to clear them of the illegal immigrants and refugees who are, according to one volunteer, "overrunning" the country. RELA makes tens of thousands of arrests every month, herding people into vans and depositing them in detention centres to be – unless they can somehow find the money to pay the bribes extorted by officials – flogged and deported.

The flogged and repatriated are the lucky ones. The unlucky ones (disproportionately Burmese refugees who have fled the tyrannical regime in their native land, but are viewed by RELA as a particular drain on resources) are, according to the testimony of victims who returned and the families of those who have not, sold into slavery.

A woman called Rahima and her family were captured by gangsters as they tried to cross the border. One gangster of them suffocated her 18-month-old son to stop him crying and sold the rest of them to a factory. A man called Abul told how they demanded £1,000 to free his wife and three children. He couldn't pay, so a fourth son went out to find them. None has been seen since.

The Malaysian government says the stories are untrue. The welts and scars on prisoners' backs, Rahima's grief and Abul's stricken face give the lie to that. Another fine portrait of an ugly beast from Unreported World.